Question:medium
Patient not unable to make "OK" Sign. Which muscle is involved?
Patient not unable to make "OK" Sign. Which muscle is involved?
Show Hint
The “OK” sign needs the fingertip (distal) joint to bend; which long flexor reaches the distal phalanx of the index finger?
Updated On: Jun 22, 2026
- FDS
- FDP
- FIC
Show Solution
The Correct Option is B
Solution and Explanation
Reason from the joint movements first.
What forms the "O": The pulp-to-pulp tip pinch of the thumb and index finger needs the very last (distal) joints to bend - the thumb IP joint and the index DIP joint. If those distal joints cannot flex, the patient instead pinches with the pads flat, producing the abnormal "pinch sign" of anterior interosseous nerve palsy.
Muscle for each joint: Thumb IP flexion $\rightarrow$ flexor pollicis longus. Index DIP flexion $\rightarrow$ flexor digitorum profundus. The flexor digitorum superficialis stops at the middle phalanx, so it only bends the PIP joint and is not involved.
Picking the option: Among FDS, FDP and FIC, the muscle that flexes the distal joint of the index finger is the flexor digitorum profundus. FDS would only flatten the PIP, and FIC is not the relevant muscle.
Clinical tie-in: Because the same nerve (AIN) drives FPL and the index FDP, the "OK" sign is the bedside screen for AIN function.
Answer: Flexor digitorum profundus - option B.
What forms the "O": The pulp-to-pulp tip pinch of the thumb and index finger needs the very last (distal) joints to bend - the thumb IP joint and the index DIP joint. If those distal joints cannot flex, the patient instead pinches with the pads flat, producing the abnormal "pinch sign" of anterior interosseous nerve palsy.
Muscle for each joint: Thumb IP flexion $\rightarrow$ flexor pollicis longus. Index DIP flexion $\rightarrow$ flexor digitorum profundus. The flexor digitorum superficialis stops at the middle phalanx, so it only bends the PIP joint and is not involved.
Picking the option: Among FDS, FDP and FIC, the muscle that flexes the distal joint of the index finger is the flexor digitorum profundus. FDS would only flatten the PIP, and FIC is not the relevant muscle.
Clinical tie-in: Because the same nerve (AIN) drives FPL and the index FDP, the "OK" sign is the bedside screen for AIN function.
Answer: Flexor digitorum profundus - option B.
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